1. Field of the Invention
The present invention relates to a Master Oscillator Power Amplifier (MOPA) light source.
2. Related Background of the Invention
A MOPA light source employs an optical amplifier to amplify seed light outputted from a seed light source, and outputs the amplified light. For example, a semiconductor laser is used as a seed light source, and an optical fiber amplifier is used as an optical amplifier.
Such a MOPA light source has a high degree of freedom of various conditions, such as the pulse width of output light and the repetition frequency, and so is attracting attention as a pulsed light source for laser processing, optical measurement, and various other applications. In the MOPA light source, the pulse period is not limited by the device hardware structure, as in mode-locked designs.
A MOPA light source having such characteristics is used in fine laser processing and other applications. In this case, there exists the demand that pulsed laser light irradiate only the portion to be processed, and that laser light not irradiate other portions. For example, in FIG. 1, a processing process is shown in which holes are opened by irradiating a printed board A, which is the object for processing, with laser light L. In this case, laser light L has to be irradiated positions at which holes are to be opened. However, the movement path (indicated by a dashed line in the figure) from the position at which a certain hole has been opened to the position at which the next hole is to be opened may become pads on which electronic components may be mounted, or may become a circuit pattern forming extremely fine wiring, and damage must not be imparted by the laser light irradiation.
As a MOPA light source which addresses such demands, there are Q-switch devices such as acoustooptic switches, and devices used in processing the second harmonic generation (SHG) and third harmonic generation (THG) occurring due to wavelength conversion using LBO, PPLN, or another nonlinear optical crystal. See Nan Ei Yu, Sunao Kurimura, Yoshiyuki Nomura, and Kenji Kitamura, “Stable High-Power Green Light Generation with Thermally Conductive Periodically Poled Stoichiometric Lithium Tantalate”, Jpn. J. of Appl. Phys., Vol. 43, No. 10A, 2004, pp. L1265-L1267 (Document 1).
A MOPA light source comprising a Q-switch outputs only weak amplified spontaneous emission (ASE) light when not in pulsed oscillation. Consequently, there is little concern that processing may occur in intervals between two holes.
As shown in FIG. 2, a MOPA light source which performs wavelength conversion using a nonlinear optical crystal generates a harmonic component only in the pulse portion in the fundamental light wave. Hence, when a dichroic mirror or similar is used to irradiate an object for processing with only a harmonic generation component, while not irradiating the object for processing with the fundamental wave component then there is no concern that processing may occur in intervals between two holes.